Conventionally, in longitudinally seam-welding, for manufacturing a welded steel pipe, the groove or cleft of an O-shaped plate (hereinafter called "pipe-blank"), formed into a cylindrical shape by a forming process such as the U-O process (abbreviation of the U-ing/O-ing process) and the bending roll process, as shown in the schematic side view given in FIG. 1 for example, it is the usual practice of welding to employ an inside welding machine equipped with a welding torch 1 attached to the free end of a boom 2 having a length at least equal to that of a pipe-blank 3 to be welded, the fixed end of said boom 2 being fixed to a carriage 4; to insert said boom 2 into said pipe-blank 3 in advance; and, to longitudinally seam-weld said pipe-blank 3 from the inside along the groove or cleft with a consumable electrode 5 fed through said welding torch 1, while moving said boom 2 by said carriage 4, together with a cable 9 serving to supply welding current to said consumable electrode 5, in the withdrawal direction from said pipe-blank 3, i.e., in the welding direction as indicated by an arrow in the drawing. An inside welding machine equipped with two welding torches 1 and 1, leading and trailing, and two consumable electrodes 5 and 5 is represented in FIG. 1. However, an inside welding machine may have one welding torch and one consumable electrode, or it may be provided with more than two welding torches and more than two consumable electrodes.
In the aforementioned conventional welding method of a pipe-blank, when adopting a welding process comprising using direct electric current as the welding current with a consumable electrode as the anode, such as the reverse-polarity GMA welding process (GMA welding process is the abbreviation of the gas metal arc welding process which comprises carrying out welding while shielding a molten metal and a welding arc produced in the space between the base metal and the consumable electrode from open air with shielding gases such as an inert gas and a carbon dioxide gas), as shown in the partially enlarged schematic drawing given in FIG. 2, the welding arc 6 from the consumable electrode 5 is deflected toward the upstream side of the welding direction as indicated by an arrow in the drawing, i.e., in the opposite direction to that of welding, and takes the form as if it is drawn in by the molten metal 7. When the welding arc 6 takes the form as if it is drawn in by the molten metal 7 as mentioned above, the plasma jet stream produced at the tip of the consumable electrode 5 is also deflected toward the molten metal 7 and acts on said molten metal 7 as a dynamic pressure. This pushes said molten metal 7 away toward the upstream side of the welding direction, i.e., in the opposite direction to that of welding. As a result, the space below the consumable electrode 5 becomes substantially dry without molten metal, thus impairing the affinity between molten droplets from the consumable electrode 5 and the base metal at the groove of the pipe-blank 3. Welding defects such as undercut of bead, humping bead and lack of fusion of base metal thus tend to easily occur. In addition, frequent occurrence of boiling and spattering in the molten metal 7 tends to result in a deteriorated appearance of the weld bead. When the welding arc 6 is deflected as described above, furthermore, the tip of the consumable electrode 5 is melted only on one side as shown in FIG. 2. Under such circumstances, the transfer mode of molten droplets from the consumable electrode 5 cannot be a desirable spray transfer, but takes an undesirable mixed form of globular transfer and short-circuit transfer. As a result, coarse spatters are splashed with a crackling short-circuiting noise and are deposited on the weld bead surface, thus leading to a deteriorated appearance of the weld bead. What is worse, splashed spatters are deposited on the opening at the tip of the shielding nozzle (not shown) of the welding torch to disturb the gas shield and entangle the air. In this case, it may practically be impossible to carry out welding.
The above-mentioned deflection of the welding arc toward the upstream side of the welding direction, i.e., in the opposite direction to that of welding and the resulting irregular weld bead and welding defects are not limited only in the case of the conventional GMA welding process, but occur also in the case of the submerged arc welding process using direct electric current as the welding current. In both cases, it has been difficult to obtain a beautiful and sound weld free from welding defects.